1. Field of the Invention
The present invention relates to a circuit for detecting a synchronizing signal.
2. Description of the Prior Art
In the prior art, an apparatus for reproducing digital audio data recorded in an optical disk is known as an example to which a circuit for detecting a synchronizing signal according to the invention can be applied.
The reproduced signal of the digital audio data has a frame structure divided by a frame synchronizing signal. The reproduced signal is subjected to EFM (Eight-Fourteen) modulation, and in order to demodulate this, data within the frame must be distinguished with respect to the frame synchronizing signal. Therefore a circuit for detecting a synchronizing signal is installed.
As a matter of fact, the reproduced signal from the disk may include a signal similar to the frame synchronizing signal generated due to a flaw of the disk or the like, or a lack of the frame synchronizing signal may be produced. In order to deal with this problem, a protection circuit of the frame synchronizing signal is required. In such a detection and protection circuit of the synchronizing signal, for example, as disclosed in Japanese patent application laid-open No. 58-98813, protection by a window is performed in order to prevent detection of a signal other than the frame synchronizing signal from being mistaken for the synchronizing signal, and insertion protection is performed in order to deal with the lack of the frame synchronizing signal. Also, if the insertion protection continues a prescribed number of times, the window is opened until the reproduced synchronizing signal is obtained. When the window is opened, if the reproduced synchronizing signal is obtained, the window is immediately closed. In the detection and protection circuit of the synchronizing signal in the prior art, however, a bit clock formed in the PLL is used to form the window signal. Since the bit clock is synchronous with the reproduced signal, it varies in case the reproduced signal is disturbed by a flaw of the disk or the like. As a result, if the window is closed by an incorrect reproduced synchronizing signal, the timing of the window signal may be shifted, even when the correct frame synchronizing signal is obtained.